1. Field of the Invention
The present invention relates to a mass spectrometer, and more specifically to an ion-optical system for converging ions and transporting the ions to a subsequent stage in a mass spectrometer.
2. Description of the Related Art
Generally, in a liquid chromatograph mass spectrometer (LC/MS) which is a combination of a liquid chromatograph (LC) and a mass spectrometer (MS), an atmospheric pressure ionization process, such as an electrospray ionization (ESI) process or an atmospheric pressure chemical ionization (APCI) process, is used. In this spectrometer, while an ionization chamber is in an atmosphere of approximately atmospheric pressure, an analysis chamber internally equipped with a detector and a mass spectrometer, such as a quadrupole mass filter, is required to be maintained in a vacuum state. For this purpose, a differential evacuation system comprising one or more intermediate vacuum chambers provided between the analysis chamber and the ionization chamber is used for increasing vacuum in a stepwise manner.
FIG. 6 shows a fragmentary schematic block diagram of a conventional LC/MS (see, for example, Japanese Patent No. 3379485). This mass spectrometer comprises an ionization chamber 11 provided with a nozzle 12 connected to a column outlet end of a LC (not shown), an analysis chamber 21 internally equipped with a quadrupole mass filter 22 and a detector 23, and a first intermediate vacuum chamber 14, and a second intermediate vacuum chamber 18. The first and second intermediate vacuum chambers 14, 18 are located between the ionization chamber 11 and the analysis chamber 21, and separated from each other by a partition wall. The ionization chamber 11 and the first intermediate vacuum chamber 14 are communicated with each other only through a solvent extraction pipe 13 having a small diameter, and the first and second intermediate vacuum chambers 14, 18 are communicated with each other only through a skimmer 16 having a top formed with a passage hole (orifice) 17 having an extremely small diameter.
An internal space of the ionization chamber 11 serving as an ion source is maintained in an atmosphere of approximately atmospheric pressure (about 105[Pa]) by vaporized molecules of a sample solution continuously supplied thereto from the nozzle 12. Then, an internal space of the first intermediate vacuum chamber 14 as a second stage is evacuated to a low vacuum state of about 102[Pa] by a rotary pump 24. Further, an internal space of the second intermediate vacuum chamber 18 as a third stage is evacuated to a medium vacuum state of about 10−1 to 10−2 [Pa] by a turbo-molecular pump 25, and an internal space of the analysis chamber 21 as the last stage is evacuated to a high vacuum state of about 10−3 to 10−4 [Pa] by another turbo-molecular pump 26. That is, a multistage differential evacuation system adapted to increase vacuum stepwise from the ionization chamber 11 to the analysis chamber 21 on a chamber to chamber basis is formed to allow the internal space of the analysis chamber 21 as the last stage to be maintained in a high vacuum state.
An operation of this mass spectrometer will be schematically described below. A sample solution is sprayed from a tip of the nozzle 12 into the ionization chamber 11 while being electrically charged, and molecules of the sample are ionized in a course of vaporization of a solvent in the droplets. The droplets mixed with ions are drawn into the solvent extraction pipe 13 due to a pressure difference between the ionization chamber 11 and the first intermediate vacuum chamber 14. In a course of passing through the heated solvent extraction pipe 13, the solvent is further vaporized and the ionization is accelerated. A first lens electrode 15 having a plurality (four) of plate-shaped electrodes arranged in three lines in a sloped manner is disposed in the first intermediate vacuum chamber 14, to generate an electric field so as to help the drawing of ions through the solvent extraction pipe 13 and converge the ions around the orifice 16. The ions introduced into the second intermediate vacuum chamber 18 through the orifice 17 are converged by an octapole-type second lens electrode 19 comprising eight rod electrodes, and sent to the analysis chamber 21. In the analysis chamber 21, only a part of the ions having a specific mass number (mass/charge) pass through a longitudinal space of the quadrupole mass filter 22, and the remaining ions having other mass numbers diverge from the longitudinal space. Then, the ions passing through the quadrupole mass filter 22 reach the detector 23, and the detector 23 outputs an ionic strength signal corresponding to an amount of the received ions.
In the above mass spectrometer, the first lens electrode 15 and the second lens electrode 19 are collectively dubbed “ion optical system”. A major function of these lens electrodes is to converge flying ions based on an electric field, and, in some cases, additionally accelerate and send the ions to a subsequent stage. Heretofore, various configurations have been proposed for the lens electrodes. In the example illustrated in FIG. 6, the second lens electrode 19 arranged in the second intermediate vacuum chamber 18 is a multi-rod type having a plurality (while the number in this example is eight, it may be any even number, such as four or six) of columnar-shaped rod electrodes (pole electrodes) arranged to surround an ion optical axis C, as shown in FIG. 7. In this case, a voltage superimposed with a high-frequency voltage having the same DC voltage and an inversed phase is applied to the adjacent rod electrodes. Thus, ions introduced in an extension direction of the ion optical axis C travel while being vibrated at a given frequency by a high-frequency electric field generated in the rod electrodes. In the multi-rod type lens electrode, there has also been known a square pole-shaped rod electrode instead of the columnar-shaped rod electrode (see U.S. Pat. No. 6,441,370).
Generally, the rod electrode, such as the columnar-shaped rod electrode or the square pole-shaped rod electrode, has to be formed by subjecting a metal block to cutting to have a required size and accuracy. Thus, a machining cost is apt to increase. Moreover, a connection between the rod electrode and a cable for applying a voltage to the electrode has to be performed by welding or the like. This operation is troublesome and costly. Further, in the ion optical system using the rod electrodes, a communication space between an internal space surrounded by the rod electrodes and an external space becomes narrow to cause a problem about a difficulty in evacuating air from the internal space of the ion optical system, and the need for taking time to evacuate the internal space to have given vacuum.